1. Field of the Invention
The present invention relates generally to the use of dense fluids for cleaning materials. More particularly, the present invention relates to monitoring changes in the amount of contaminants present in the dense fluids as the cleaning process progresses.
2. Description of Related Art
Conventional solvent-aided cleaning processes are currently under severe scrutiny due to problems with air pollution and ozone depletion. In addition, recent environmental concerns mandate that many of the organic solvents used in these processes be banned or their use severely limited. The use of dense phase gases for cleaning a wide variety of materials has been under investigation as an alternative to the above-mentioned solvent based cleaning processes. A dense phase gas is a gas compressed under either supercritical or subcritical conditions to liquid-like densities. These dense gases are referred to as dense fluids. Unlike organic solvents, such as n-hexane, or 1,1,1-trichloromethane, dense phase gas solvents exhibit unique physical properties such as low surface tension, low viscosity, high diffusivity and variable solute carrying capacity.
The solvent properties of compressed gases are well known. In the late 1800's, Hannay and Hogarth found that inorganic salts could be dissolved in supercritical ethanol and ether (J. B. Hannay and H. Hogarth, J. Proc. Rov. Soc. (London, 29, p. 324, 1897). By the early 1900's, Buchner discovered that the solubility of organics such as naphthalene and phenols in supercritical carbon dioxide increased with pressure (E. A. Buchner, Z. Physik. Chem., 54, p. 665, 1906). Within forty years Francis had established a large solubility database for liquified carbon dioxide which showed that many organic compounds were completely miscible (A. W. Francis, J. Phys. Chem., 58, p. 1099, 1954).
In the 1960's there was much research and use of dense gases in the area of chromatography. Supercritical fluids (SCF) were used as the mobile phase in separating non-volatile chemicals (S. R. Springston and M. Novotny, "Kinetic Optimization of Capillary Supercritical Chromatography using Carbon Dioxide as the Mobile Phase", CHROMATOGRAPHIA, Vol. 14, No. 12, p. 679, December 1981). Today the environmental risks and costs associated with conventional solvent-aided separation processes require industry to develop safer and more cost-effective alternatives.
The volume of current literature on solvent-aided separation processes using dense carbon dioxide as a solvent is evidence of the extent of industrial research and development in the field. Documented industrial applications utilizing dense fluid cleaning include extraction of oil from soybeans (J. P. Friedrich and G. R. List and A. J. Heakin, "Petroleum-Free Extracts of Oil from Soybeans", JAOCS, Vol. 59, No. 7, July 1982), decaffination of coffee (C. Grimmett, Chem. Ind., Vol. 6, p. 228, 1981), extraction of pyridines from coal (T. G. Squires, et al., "Supercritical Solvents. Carbon Dioxide Extraction of Retained Pyridine from Pyridine Extracts of Coal", FUEL, Vol. 61, November 1982), extraction of flavorants from hops (R. Vollbrecht, "Extraction of Hops with Supercritical Carbon Dioxide", Chemistry and Industry, Jun. 19, 1982), and regenerating absorbents (activated carbon) (M. Modell, "Process for Regenerating Adsorbents with Supercritical Fluids", U.S. Pat. No. 4,124,528, issued Nov. 7, 1978).
Electro-optical devices, lasers and spacecraft assemblies are fabricated from many different types of materials having various internal/external geometrical structures which are generally contaminated with more than one type of contamination. These highly complex and delicate systems are generally classified together as "complex hardware". Conventional cleaning techniques for removing contamination from such complex hardware requires that the hardware be continually cleaned during assembly. The use of supercritical fluids, such as carbon dioxide is particularly well-suited for cleaning such complex hardware.
Supercritical fluid cleaning systems operate at high temperatures and pressures. As a result, real time monitoring of the cleaning process is difficult. In current systems, parts and materials are cleaned or extracted for a period of time and then removed and tested for cleanliness. If the part is still contaminated, it must be reintroduced into the system and recleaned. In order to avoid having to reclean numerous parts, the parts are typically left in the system much longer than necessary to insure adequate cleanliness. This, of course, results in a great deal of unnecessary cleaning, waste of time, and increased costs.
Experimental supercritical cleaning has used U-V fluorescence detection for in-situ monitoring as described, for example, by Keith M. Motyl, in "Cleaning Metal Substrates Using Liquid/Supercritical Fluid CO.sub.2," NASA Tech Briefs, December 1990. However, this technique provides a sensitivity on the order of parts per million (ppm) which is not sufficient for precision cleaning requirements. Sensitivity on the order of parts per billion (ppb) is required for precision cleaning.
It would be desirable to provide a system for monitoring supercritical fluid cleaning systems to determine when the particular part has been completely cleaned or when the extraction of desired materials has been completed. Such a monitoring system should be simple, efficient and capable of being used in monitoring a wide variety of cleaning/extraction processes utilizing supercritical fluids. This system should also have a sensitivity on the order of parts per billion in order to be useful for precision cleaning.